Expandable sealants have been known and used for some years. Basically, these sealants are prepared in two forms. The first form is commonly referred to as "pumpable sealants" in which the composition is in the form of a mastic or paste. These sealants can be heated to reduce their viscosity so that it may then be possible to pump the sealant through a line to the desired location. The sealant is then heated to a sufficiently high temperature causing it to expand and cure and since they are pumped into place they are not required to flow during the expansion and curing cycle.
Additionally, these materials do not have to be extrudable nor do they have to possess any "green strength" since they are not intended to be handled but only pumped into position. In the past such materials have been formulated from any number of polymers some of which contained styrene-butadiene diblock copolymers, and terpolymers mixed with polyvinyl chlorides and other rubber materials.
The second type of sealants, and the ones to which this invention is directed, are known as "hot melt" sealants. These sealants are characterized in that unlike the pumpable materials, these have good "green strength" and are capable of being formed into shapes, typically by extrusion, so that they might easily be placed in a preliminary position and then when heated the sealant would flow into the desired position, expand and cure. Again these materials have been formulated using styrene butadiene diblock copolymers and styrene butadiene divinylbenzene terpolymers, among other polymers.
One of the more common uses for the hot melt sealants have been in the automobile industry to seal or fill in gaps between mating pieces of the automobile. For instance these sealants are used in the area where the windshield meets the frame of the car where the fit may not be perfect and will result in a gap which may permit dust, dirt, noise or other material to enter through this gap or it may form a trough where water may collect and eventually cause rust to develop.
Both of these sealant types have in the past been manufactured to respond (expand, cure or flow) at temperatures in excess of 300.degree. F. The reason for this is that these sealants are applied to the car prior to their being passed through the paint bake cycle in the factory which until recently has been operated at temperatures in excess of 300.degree. F., typically temperatures of about 325.degree. F. to 350.degree. F.
However, due to the high cost of energy and the increased emphasis on cost reduction there has been a big push in the automobile industry to reduce the operating temperatures of the paint ovens and consequently the operative temperatures of these sealants. Currently the manufacturers intend to operate these ovens in the temperature range below 300.degree. F., and ideally at temperatures in the range of 225.degree. F. to 275.degree. F., up to nearly 100.degree. F. below the current levels. This is going to necessitate the modification or alteration of these different sealants so that they will be useful at these lower temperatures.
This lowering of the reaction temperature of these complicated sealants is not a simple matter. Contained in the sealant is a blowing agent to cause the sealant to expand, a curing agent to cause the sealant to cure and additionally the sealant must flow properly during the heating cycle to seal effectively. All of these three reactions happening simultaneously to one extent or another during the bake cycle. Therefore, a balance between the rapidity of the flow or the thinning out of the sealant, and the expansion as well as the timing of the cure are important for the sealant to operate properly. A sealant that flows to rapidly will not have the required body to capture the blow and expand, while the cure can not be too rapid or it will reduce the flow of the sealant and could impair the expansion of the sealant due to the increase in viscosity.
Therefore what is needed in this art is a new hot melt sealant which will flow, expand and cure at temperatures below the current operating temperatures of 325.degree. F. to 350.degree. F.